import numpy as np
import pandas as pd

data = pd.DataFrame([
    [1., 6.5, 5.],
    [2., np.NAN, np.NAN],
    [np.NAN, np.NAN, np.NAN],
    [np.NAN, 9, 3.],
], columns='cola,colb,colc'.split(','),index='row1,row2,row3,row4'.split(','),)

print(data,'\n')

print(data.isna().any(axis = 'index'),'\n')

data.fillna(data.mean())

